Audio power amplifiers are widely used in various applications to amplify audio signals and drive speaker loads. Many of the medium to high power audio amplifiers employ power integrated circuits for lower cost and better integration. In some applications, such as automotive, the power integrated circuits are subjected to very severe conditions where the outputs can potentially short to various voltage potentials available in the vehicle environment. FIG. 1 shows an illustration describing a nominal listening level in a class H audio amplifier in the prior art. The power IC outputs are shown as arbitrary waveforms. The power supply rails VDD and VSS track the audio outputs as shown in the figure. The battery voltage is not within the rails that the power IC sees. However, the outputs or speaker terminals can short to the battery voltage.
Class H audio amplifiers using power integrated circuits have been developed for better efficiency compared to class AB amplifiers. The power integrated circuits in these class H amplifiers are sometimes subjected to shorts at the board and system level while the IC is amplifying audio signals. Unlike previous class AB amplifiers, the power integrated circuit outputs in a class H system could short to voltage potentials that are unrelated to its power supply rails, as illustrated in FIG. 1. This creates a major challenge in fast detection of shorts and protection of the power integrated circuit.
Prior solutions for short circuit protection in power integrated circuits focus on sensing current through the output driver devices and flagging an overcurrent condition if the current goes beyond a pre-determined threshold voltage. FIG. 2 is an illustration of the block diagram of the overcurrent scheme implemented in an exemplary power integrated circuit in the prior art. During a short circuit event usually the current through the output driver device(s) will increase drastically to cause an overcurrent event. There are two disadvantages to using overcurrent protection alone as a mechanism to protect a class H power integrated circuit against short circuit. First, in certain short circuit situations, the current through the actual output devices is lower than the overcurrent threshold. An example is shown in FIG. 3 where the majority of the high current from the battery will flow through the body diode of the high side device (D1 or D3 in FIG. 2) which is not detected by overcurrent protection. Second, the overcurrent protection mechanism is not able to determine the exact nature of the fault and report it back to system controller.
FIGS. 3(a)-(d) are an illustration of exemplary simulation results for a speaker terminal short circuit to battery voltage in the prior art. In this simulation test bench, the conditions were made similar to the one described in FIG. 1, which corresponds to a nominal listening level. The short to battery is applied at 6 milliseconds to OUTx(+), and the output is kept shorted for the remainder of the simulation. The current drawn by device MN+ during a short to battery is shown in FIG. 3(a). This output driver device carries most amount of current in comparison to the other three output devices because of amplifier feedback in response to output shorted to battery. Even though there is a high peak transient current, the current sense voltage Vocm+ shown in FIG. 3(b) is not high enough to generate an overcurrent flag. The current sense mechanism does not have enough bandwidth to capture the very fast initial current transient in device. The current drawn by parasitic body diode device D1 is shown in FIG. 3(c). This diode is forward biased during the short circuit. The overcurrent protection or current sense mechanism is not able to sense current through the body diode D1. The current through diode D1 as shown in FIG. 3(c) is much higher in magnitude than output driver device MN+ and is high enough to cause destruction in diode D1 and output drive MP+. FIG. 3(d) shows the current sense voltage Vocp+, which does not capture the high current through the diode D1. Based upon simulation results in FIG. 3(a)-(d), it can be concluded that overcurrent protection is not sufficient in protecting the power integrated circuit under all possible short circuit conditions in a vehicle environment.
Another approach for short circuit protection of power integrated circuits is a high side and low side sense current comparison. This approach has the advantage of detecting short circuit conditions even in scenarios where driver device current does not exceed overcurrent trip threshold. The current sense voltage for opposing hi-side and lo-side driver device is compared. If the currents are significantly imbalanced, then the output of the difference amplifier will be high enough to cause the comparators to trip indicating a short circuit. The disadvantages of this method are: 1) this method works for a fully differential bridge configuration, but not a speaker driven in a single ended half bridge configuration; 2) it is very difficult for this method to distinguish between short to the negative side of the power source (−VP) and short to ground, so there is the possibility of inaccurate fault reporting with this method; and 3) this method does not detect a shorted speaker condition.
Thermal modeling of output driver device based upon current sense and voltage drop from supply voltage has also been used in the past for protection against short circuit conditions. However they suffer the same disadvantages as overcurrent protection methods.
Thus, an unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.